Nuclear weapons are seen as the last resort of Israel's security, the so-called "Samson option" to be used in desperation - like the biblical character who died with his enemies when he brought down the temple on the heads of the Philistines.

Developed secretly from 1956 after France built a nuclear reactor at Dimona in the Negev desert, the weapons were seen by Israel's first generation of leaders as designed to prevent a second Holocaust - an argument that was translated into a formidable arsenal outside any international controls. Seymour Hersh, the American writer, has reported that the words "Never Again" were welded, in English and Hebrew, on to the first Israeli nuclear warhead. Apocryphal or not, the story hints at the thinking behind the programme.

Israel, unlike Saddam Hussein's Iraq, never signed the nuclear non-proliferation treaty, the 1970 agreement which allows countries to develop civilian nuclear power in exchange for forgoing weapons. These are supposed to be the preserve of the five permanent members of the UN security council. In recent years India, Pakistan and North Korea have swelled the ranks of the weapons states, but unlike them Israel has never come out of the nuclear closet, preferring a policy of so-called nuclear ambiguity - keeping its enemies guessing.

Israel's official line has always been that it would not be the first to use nuclear weapons in the Middle East. But fears about the nuclear ambitions of Iran - led by a Holocaust-denying President Ahmadinejad - have reinforced domestic support (and perhaps international tolerance) for retaining its arsenal. In diplomatic terms, this has long been a no-go area for the US, Britain and others.

The closest it has come to using a nuclear weapon was in October 1973, when Egypt and Syria caught Israel by surprise and caused it heavy losses in the first days of the Yom Kippur war. By the mid-1980s when whistleblower Mordechai Vanunu, a former technician at the Dimona reactor, gave his sensational inside story to the Sunday Times, the expert assessment was that Israel had up to 200 nuclear warheads and the ability to "deliver" them by plane, missile and submarine. If true, that makes a country of 7 million people the world's fifth or sixth ranking nuclear power.

In 1991, when Saddam launched his Scud missiles at Israel, there were fears he might be able to hit the heavily-guarded Dimona reactor. This month there were jitters when a Palestinian suicide bomber struck a shopping centre in the town.

In September Israeli warplanes launched a raid on northern Syria on a site rumoured to have been a nuclear reactor. The attack was variously interpreted as being designed to restore Israel's deterrent capability and to send a signal to Iran about what could happen if it did pursue a nuclear weapons programme.